In a hot-rolling process, a hot-rolled steel strip which has passed through a finishing rolling process (hereinafter, also referred to as “steel strip”) is transported from a finishing rolling mill to a down coiler. During this transportation, the steel strip is cooled to a predetermined temperature by means of a cooling device formed by plural cooling units, and then, is coiled by the down coiler. At the time of hot-rolling the steel strip, the cooling manner of the steel strip after passing through the finishing rolling process to the coiling is an important factor in determining mechanical properties of the steel strip. In general, the steel strip is cooled, for example, by using water as a cooling medium (hereinafter, also referred to as “cooling water”). In recent years, the cooling is carried out in a high temperature range at a high cooling speed (hereinafter, also referred to as “rapid cooling”), for the purpose of maintaining workability and strength more than or equal to those of the conventional steel strip while reducing additional elements such as manganese in the steel strip. Further, from the viewpoint of maintaining the uniformity of cooling, there is known a method of cooling, which avoids the cooling in a state of transition boiling, which is a primary factor of nonuniformity in cooling, as much as possible, and employs cooling in a state of nucleate boiling, under which a stable cooling capability can be obtained. In general, the cooling in the state of nucleate boiling is the rapid cooling.
In the finishing rolling process, an accelerated rolling and a decelerated rolling are widely employed. A transportation speed of the steel strip on the output side of the finishing rolling mill is equal to a transportation speed up to the down coiler, and the steel strip is cooled in a state where the transportation speed changes. Therefore, in general, when the hot-rolled steel strip is cooled using rapid cooling, the cooling length and the water amount density of the cooling water are changed in accordance with an increase or decrease in the transportation speed of the steel strip, in order to achieve a target coiling temperature of the steel strip. For example, Patent Document 1 discloses a method of cooling in which, after the final finishing rolling milling, the length of the cooling zone is adjusted in accordance with an increase or decrease in the rolling speed of a hot-rolled steel plate such that the amount of decrease in temperature of the steel plate is constant within the steel plate. This method includes: a rapid cooling step of rapidly cooling the steel plate under a condition of a water amount density of 1000 L/min/m2 or more; and a slow cooling step of slowly cooling the hot-rolled steel plate after the rapid cooling step such that the steel plate is coiled at a predetermined coiling temperature of the steel plate.
Further, Patent Document 2 discloses a technique in which cooling water with a water amount density of 2.0 m3/m2min or more is supplied, and the length of a cooling zone is adjusted by independently switching ON-OFF each cooling header of a first cooling header group and a second cooling header group in accordance with an increase in the transportation speed.
Related Art Document
Patent Documents
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2008-290156
Patent Document 2: Japanese Patent Publication No. 4449991